Esters are produced by the reaction of an alcohol with a carboxylic acid or a carboxylic acid anhydride. In many instances, one or more of the starting materials may be a mixture. The carboxylic acid may be a mono or a polycarboxylic acid or the anhydride thereof. Plasticiser esters are generally produced from polycarboxylic acids or the anhydrides thereof and in particular from phthalic anhydride, cyclohexanoic dicarboxylic acid or its anhydride, adipic acid or anhydride or trimellitic acid or anhydride. Esters of benzoic acid, such as isononyl benzoate or isodecyl benzoate, are examples of plasticiser mono-esters. Esters may be produced from any alcohol, but plasticiser esters are generally produced from C4 to C13 alcohols and in particular C6 to C13 alcohols more typically C8 to C10 alcohols.
The production of esters and in particular plasticiser esters is described in U.S. Pat. Nos. 5,324,853; 5,880,310; 6,310,235 and 6,355,817. All of these patents disclose that the esterification reaction is reversible and that excess alcohol is preferably used in the esterification reaction to drive the reaction to completion. U.S. Pat. Nos. 5,324,853 and 6,355,817 suggest that the excess alcohol be collected after the esterification reaction and recycled. The present invention is particularly concerned with optimising the recycle of alcohol to the esterification reaction.
It is known from U.S. Pat. No. 5,880,310 that oxygen should be removed from alcohols used in the production of plasticiser esters. The oxygen is removed in order to improve the colour of the final ester. It is also known that esterification reaction cycle time can be reduced if the alcohol is preheated before it is fed to the esterification reactor.
No consideration has been given however to the optimisation of the recycle of alcohol. In reaction systems in which alcohol is recycled to the esterification reaction the recycle alcohol is mixed with fresh alcohol in proportions such that the appropriate level of excess alcohol is fed to the esterification reaction. However, the fresh alcohol feed is generally not 100% pure alcohol, but contains some residues from the alcohol manufacture such as olefins, paraffins and olefin oligomers. The fresh alcohol may also include for example traces of aldehydes, formate esters, di-alkyl esters and ethers, cyclic ethers, and peroxide inhibitors from the olefin feedstock used in the production of the alcohol. The alcohol manufacture is typically accomplished by either olefin oligomerisation followed by hydroformylation followed by hydrogenation, or olefin hydroformylation followed by aldol condensation followed by hydrogenation. Whichever process is used and whatever purification techniques are used, the resulting alcohol will typically contain some impurities. These impurities will generally not participate in the esterification reaction and accordingly will pass through the esterification reaction and remain in the recovered excess alcohol, and/or pass through with the crude ester product.
After esterification the crude ester will contain contaminants and requires purification. These contaminants may belong to the family of acidic residues, unreacted alcohol, catalyst residues, water and the contaminants that were already present in the alcohol feed, most of these being so-called monomeric components that are eluted in the so-called “light ends” region of the plasticiser Gas Chromatogram or GC-spectrum.
The crude esters may also contain byproducts, such as alcohol (di-alkyl) ethers, benzoate esters, mono-esters from dibasic acids, alcohol oxo acid esters, hemiacetals and vinyl ethers (these are so-called dimeric components, because they originate from a combination reaction of two molecules of the starting materials, and are often collectively called “ethers” or “intermediates” due to their elution in the plasticiser Gas Chromatogram or GC-spectrum between the monomeric light ends and the “trimeric” target product diesters). In the production of triesters, such as trimellitates, also so-called “trimeric” compounds because they combine three molecules of starting material, such materials as e.g. acetals or diesters, may be considered “intermediate” impurities. Many of these dimeric or other intermediate materials, including acetals, may become hydrolysed, in particular during later stages in the esterification process, to form odour formers such as aldehydes and/or other light ends, which often add to the so-called monomeric component part of the contaminants. Trimeric components may also add to the dimeric contaminants when they break apart, such as by hydrolysis. Any of such impurity component that is sufficiently volatile and is formed before the last recovery step of excess alcohol, may end up as a contaminant of the recycle alcohol. Other impurities may be generated as byproducts in the esterification reaction and remain in the recovered excess alcohol. These can include mono-esters in the case of di-ester production, di-esters in the case of tri-ester production, phthalide in the case of phthalate production, alcohol benzoates if benzoic acid is present as an impurity in the acid or acid anhydride, maleates if maleic acid or anhydride is present, and phthalic acid in case where phthalic anhydride is used. Also these components may show up as impurities in the recycle alcohol.
In the steps taken for removal of the excess alcohol from the crude ester product mixture, some of the product ester may also end up in the recycle alcohol. Upon recycle of the recovered excess alcohol, this product ester is not lost. This amount of ester however dilutes the recycle alcohol. These product ester compounds in the recycle alcohol may therefore also be considered as impurities.
The impurity level in the recovered excess alcohol will be considerably higher than in the fresh alcohol. Accordingly, when this recovered excess alcohol is recycled to the esterification reaction, this will cause a build up of impurities in the fresh alcohol/recycle alcohol mixture which is fed to the esterification reaction, particularly in reactions involving repeated recycles. Accordingly, a series of recycles can result in a level of impurities as high as 20 to 30% by weight in the excess alcohol recovered and recycled to the esterification reaction. This, in turn, leads to a distortion in the projected alcohol to acid molar ratio in the esterification reaction, leading to a lower, if any, excess alcohol level and hence a less complete reaction or the need for a longer reaction time. Because typically a target conversion needs to be reached in order to achieve target production economics, this results in a batch time that is unpredictable and typically understated. Therefore samples need to be taken and analysed to assure the target degree of reaction, often expressed as a target conversion, is reached. The sampling and analysis time is therefore added to the batch time, which reduces productivity.
The present invention addresses these problems.